1. Field of the Invention
The present invention relates to an electrode for electrochemical measurements in aqueous solutions, especially electrochemical measurements in water in or from the primary circuit of a nuclear reactor during operation at the temperature of the water prevailing at that time.
2. The Prior Art
In operation of nuclear reactors, corrosion of construction material in the primary circuit occurs, that is, in the circuit of a boiling water reactor where steam generated in the reactor vessel is led to a steam turbine, from there to a condenser and condensate formed in the condenser, after preheating, is returned to the reactor vessel, and in the circuit of a pressurized water reactor where water heated in the reactor vessel is led to a steam generator and from there is returned to the reactor vessel, as well as in the secondary circuit of a pressurized water reactor, that is, in the circuit where steam generated in the steam generator is led to a steam turbine, from there to a condenser and condensate formed in the condenser, after preheating, is returned to the steam generator. Of particular importance are such cases of corrosion which result in stress corrosion, which may occur, among other things, in pipe sockets with welding stresses and such as lead to oxide growth on fuel tubes and fuel channels. A corrosion progress in the primary circuit of a reactor can be monitored during operation by continuously measuring the electrochemical potential of construction material included in the primary circuit, for example in stainless steel. It can then be determined whether such changes in the corrosion environment have occurred as require that measures regarding the operation of the reactor should be taken in order to eliminate the causes of the changes. For example, the content of oxygen or the content of other corrosive substances may have become impermissibly high, resulting in increased corrosion and oxide growth. The corrosion environment in the primary and secondary circuits of a reactor during operation can also be monitored by continuously measuring the electrical conductivity of the water in the circuit. By following the conductivity, it is also possible to establish whether such changes of the reactor water have occurred as require corrections of the operation of the reactor, such as, e.g., replacement of ion exchange filters.
It is of the utmost importance that the measurements mentioned above are carried out under the conditions prevailing in the reactor in operation, since solubilities and equilibriums at other temperatures are different and therefore not representative. The measurements of the electrochemical potential of selected construction materials require a reference electrode. It is of decisive importance for the measurement results that the reference electrode is stable and reliable at the high temperatures prevailing in the reactor water. Up to now there has been used as reference electrode above all a silver-silver chloride electrode with an electrode chamber containing water with dissolved silver chloride or dissolved potassium chloride. The electrode chamber is provided with a liquid bridge which delimits the electrolytic chamber from the water into which the electrode is immersed. In the known reference electrode the silver conductor is surrounded by an electrically insulating casing consisting of polytetrafluoro ethylene. The casing may, for example, consist of a tube of polytetrafluoro ethylene which is shrunk onto the silver conductor. Also seals for walls in the autoclave or corresponding equipment, in which the electrode is inserted during measurement, as well as the electrolytic chamber, are made of polytetrafluoro ethylene. In the same way as electrodes of construction materials whose electrochemical potential is to be supervised, the reference electrode is arranged in a circuit outside the reactor vessel, through which reactor water is led which is thereafter passed to an outlet. One drawback with the polytetrafluoro ethylene is that its use involves severe sealing problems during temperature cyclings due to its limited dimensional stability and strength at the temperatures which prevail in the water in the primary and secondary circuits of reactors. The sealing problems result in the reference electrode becoming complicated in construction without it being possible to eliminate the risk of leaks arising and, thus, the risk of radioactive water leaking out. Electrodes which are used during conductivity measurements may, in an analogous manner, be provided with insulating casings of polytetrafluoro ethylene. These casings, of course, have the same weaknesses as those described for the reference electrode of silver/silver chloride.